An investigation of the Bi2O3-SO3 system with Bi2O3/SO3 ratio (3-12):1 was carried out using X-ray powder diffraction, infrared spectroscopy and impedance measurements. New bismuth oxysulfate phases Bi9SO16.5 (4.5:1) and Bi15SO25.5 (7.5:1) are found. Their structures are interpreted as fluorite-related superstructures with lattice parameters: a(0) = 11.024 Angstrom, c(0) = 28.262 Angstrom and a(0)=16.440 Angstrom, c(0)=28.835 Angstrom for Bi9SO16.5 and Bi15SO25.5, respectively. Some physical and chemical properties of these compounds were investigated including impedance and ionic conductivity. (C) 2003 Published by Elsevier Science B.V